This invention relates to a device for opening, in particular, scissor-type jack mechanism of variable length which is usually located in the space between opening bars of the hoisting mechanisms, for example a scissor-type mechanisms of jacks and lifting platforms, and which consists of a variable volume chamber connected to a controlled inlet and outlet of a fluid medium. A change of chamber volume produces a change of length of the opening device whose ends associated with arms of the cross-bars mechanism effectuate opening of the arms and thus the elevation of a platform situated on the top of a cross-bar mechanism.